1. Field
The present disclosure relates generally to avalanche photo diodes (APD), and more specifically to APDs used in arrayed devices.
2. Description of Related Art
An APD is a semiconductor photodetector that turns light into an electrical signal. A basic APD has two main regions: an absorption region and a multiplication region. Depending on the APD design, the absorption region may be on one side only of the multiplication region or on both sides. Photons that are absorbed in the absorption region generate an electron-hole pair. The electron (if absorption region is p-type) drifts or is carried by a low-level electric field to the multiplication region. The strong electric field in the multiplication region accelerates the electron to a point where the electron has enough energy to generate more electron-hole pairs through impact ionization within the multiplication region. Depending on the semiconductor and APD design, electrons, holes, or both that reach the multiplication region may be accelerated to the point that they may create additional electron-holes pairs. This process may continue indefinitely or until all carriers are swept out of the multiplication region without generation of additional carriers. Through this process of impact ionization, one electron-hole pair may generate hundreds or thousands more (or significantly more) electron-holes pairs. As the electrons and holes reach the terminals, an electrical current is generated, which may be detected and measured.
The number of electron-holes pairs generated from a single absorbed photon (i.e., the APD gain) varies based on the design and operating point of the APD. There are two main modes of operation for APDs: analog mode and Geiger mode.
In analog mode, the gain of the APD is a function of the APD structure and the reverse bias applied to the APD. Typically, the higher the reverse bias, the higher the gain. For example, an APD may exhibit a gain of 20 at 125V and a gain of 300 at 185V. Gains of up to 1000 or more are possible depending on the materials, the manufacturing process, and the design of the APD. Note that in analog mode, the electrical field in the multiplication region is not strong enough to create electron-hole pairs indefinitely. Eventually all of the carriers will be swept out of the device and the current will drop to zero until another photon generates an electron that reaches the multiplication region.
In Geiger mode, the APD is reverse biased higher than in analog mode and above the APD breakdown voltage. The reverse bias creates a very strong electric field that may result in a gain of 105 or 106 or the impact ionization process described above may even “latch” the APD and become self-sustaining. If the process becomes self-sustaining, then as long as the electric field is maintained, electron-hole pairs will continue to be generated and a current will continue to flow through the APD.
In addition to the operational mode of the APD, the reverse bias applied to the APD may also determine the probability of a generated electron or hole creating a detectable signal through an impact ionization process. In some APDs, the reverse bias may be set to reduce the probability of one carrier (e.g, a hole) triggering the APD as compared to the probability of the other carrier (e.g., an electron) triggering the APD.
If a photon generates a self-sustaining impact ionization process, either by a generated electron or hole, an APD may use a quench to reset itself. In particular, the APD may use a passive or active quench.
A passive quench may be implemented with a high value resistor connected in series between the cathode or the anode of the APD and the voltage source supplying the reverse bias to the APD. Once a photon generates an electron that triggers a self-sustaining impact ionization process in the multiplication region, a current starts to flow through the APD. The current will cause a significant voltage drop through the high value resistor. The voltage drop across the high value resistor will reduce the electric field in the multiplication region, which will reduce the chance that electrons and holes in the multiplication region will create additional electrons-hole pairs. Once the electric-field drops low enough, the impact ionization process will terminate and the APD will reset because the high value resistor will no longer have a voltage drop across it.
An active quench uses a quench circuit to detect a latched APD. Once the quench circuit detects that the APD has latched, the circuit may disconnect the APD from the voltage source or reduce the reverse bias being applied to the APD. Either of these actions will reduce the electric field in the multiplication region. Once the electric field drops low enough, the self-sustaining impact ionization process will terminate and the APD will stop conducting. The quench circuit may then restore the reverse bias to the APD to reset the APD for the next photon.
Other than the gain, there are several other important parameters that describe the performance of an APD. For example, quantum efficiency is the probability that a photon will generate an electron-hole pair in the absorption region and the electron or hole will reach the multiplication region and initiate an impact ionization process that does not terminate prematurely. Dark counting rate is the rate at which non-photon generated carriers initiate the impact ionization process. It is impossible to differentiate these signals from those generated by photons.
In addition to using APDs as individual discrete devices, multiple APDs may be used in integrated arrays. Arrayed APDs may be useful in, for example, imaging applications. A silicon photomultiplier (SiPM) is an example of a device using an array of APDs. In arrays of APDs, each APD may be known as a pixel of the array.
In addition to the performance parameters of the individual APDs, other performance parameters may be relevant to arrayed APDs. For example, cross-talk is the probability that an impact ionization process in one APD will trigger an impact ionization process in a neighboring APD.
Additional descriptions of APDs may be found in U.S. Pat. No. 7,759,623 and U.S. patent application Ser. No. 11/725,661, filed Mar. 20, 2007, published as US Patent Publication No. 2008/0012087 assigned to the assignee of the present invention, both of which are incorporated herein by reference in their entirety for all purposes.